DE102017130924B3 - hybrid filter - Google Patents

Abstract

The invention combines two filter technologies in a single device using the same substrate therefor. Arranged on this substrate is a filter circuit having a ladder-type or a cross-member array of series and parallel impedance elements to provide a hybrid filter with, for example, a bandpass function. The impedance elements are selected from BAW resonators and LC elements.

Description

Acoustic filters conventionally have a branch or a cross-member structure. In the ladder type structure, series resonators and shunt resonators are combined to provide a desired filtering function, e.g. B. a bandpass function to produce. In cross-link structures, two row signal lines are connected to series resonators with parallel branches, in each of which parallel resonators are arranged. An achievable bandwidth of such filter structures can be estimated to be about twice the pole-to-zero distance PZD of the resonator used. Standard topologies of such filter structures use SAW resonators or BAW resonators, both of which are comparable in terms of their PZD. BAW resonators are preferred at frequencies greater than 2 GHz and in all cases where high power signals are to be processed because of their higher power resistance compared to SAW resonators.

For recent high frequency, high frequency applications, the requirements exceed the achievable performance of these standard structures. Accordingly, non-standard topologies are necessary.

LC elements can also be used to form filter structures. However, although the bandwidth of LC filters is higher, the passband that is achievable has flanks that are less steep than those of SAW or BAW acoustic resonators due to the lower quality factor.

In order to further improve the performance of the critical edge of the filter passband, acoustic resonators are used in combination with LC elements to improve the slope of the edge, while maintaining the high bandwidth.

A new approach to improving the quality of LC elements is in the published patent application US 2017/0077079 A1 described. There, a glass substrate is used to build high Q LC elements in a multilayer metallization embedded in a dielectric. Vias are used to connect different metallization levels and improve the integration factor. In the following context, these LC elements are called POG elements (Passive On Glass - POG elements) and the associated manufacturing process is called a POG process.

On the other hand, BAW filters are commonly formed on semiconductor wafers for possible integration of semiconductor devices therein.

Therefore, forming a hybrid filter by combining a BAW structure and an LC structure leads to the disadvantage of two different and accordingly separate substrates for realizing the combination. There are two This necessary, resulting in a significant space consumption, which may be critical in mobile or handheld devices.

It is an object of the present invention to provide a filter which overcomes the above-mentioned problems.

This and other objects are achieved by a hybrid filter according to claim 1. Further embodiments of the invention are the subject of further claims.

The general idea of the invention is to combine the two technologies on a single device using the same substrate for it. Arranged on this substrate is a filter circuit having a ladder-type or a cross-member arrangement of series and parallel impedance elements. The impedance elements are selected from BAW resonators and LC elements. The LC elements comprise at least one metal-insulator-metal capacitor (MIM capacitor) and a coil. The LC elements are formed from a multilayer metallization. Each level of multilayer metallization comprises at least one LC element embedded in a dielectric. LC elements which are formed in the same metallization plane are electrically connected by conductor tracks. These lines can be formed in an integrated process parallel to the LC elements. LC elements, which are formed in different metallization levels, are connected by vias. These traces and vias may also be used to connect the LC elements to the BAW resonators on the same wafer, depending on whether they are located in the same or different levels of the device.

This new approach leads to a reduction of the necessary overhead, that is to a reduced connection cost, a reduced number of solder pads and smaller printed conductors and therefore to a reduced resistance and correspondingly reduced electrical losses. In addition, area consumption is reduced as compared to a solution using two different wafers as substrates.

A preferred substrate for realizing LC elements thereon is a glass wafer. Those wafers are non-conductive and can be provided with a flat and even surface. The glass substrate is also suitable for building BAW Resonators on it very beneficial. For example, instantaneous BAW processing uses silicon wafers that are specially treated to reduce their conductance. A substrate made of a glass wafer also shows a reduction of a parasitic conductance.

However, if any BAW process were to require a silicon wafer that is compatible with a standard semiconductor process, the POG process may also be performed on such another substrate commonly used for BAW devices. In any of the two methods, only one substrate is necessary and the necessary area for the entire device is reduced to the area of the larger of the two structures formed in different technologies.

In a preferred embodiment, the LC elements are implemented as passive elements on glass (POG), as described in the already mentioned US patent application. Therefore, the LC elements are formed on the glass substrate in two or more metallization planes embedded in a dielectric. The dielectric may be selected from an organic material or a dielectric ceramic material, such as silicon oxide, and similar materials.

Due to the complete embedding of the LC elements within a dielectric layer, the surface thereof may be planarized in a later step to provide a planar surface that is helpful in arranging BAW structures thereon. A further advantage is achieved if the POG elements are covered with an additional planar dielectric layer disposed above the LC elements. The BAW resonators are formed directly on the plane. The BAW resonators may be formed in any of the two configurations currently used. They can be formed in a SMR (Solidly Mounted Resonator) technology or as a membrane-type resonator suspended above an air gap.

SMR devices require a Bragg mirror to hold the acoustic energy within the BAW device. Such an acoustic Bragg mirror is formed as a set of at least two alternating layers with relatively high and relatively low acoustic impedance, that is usually made of a metal and a dielectric.

In an alternative embodiment, the BAW resonators are formed on the surface of the substrate. At least one upper acoustic Bragg mirror is formed on the BAW resonators, connected together in a ladder-type or cross-link structure. This mirror serves as an interface for LC elements formed on this mirror. In this way, the acoustic energy is kept within the BAW resonators and the mass loading of the structures deposited on the mirror has no effect on the acoustics of the BAW resonators.

In another embodiment, the LC elements and the BAW resonators are formed on opposite surfaces of the same substrate. In this variant of the filter circuit, the LC elements can be electrically connected to the BAW resonators by vias guided through the substrate. One of the two top surfaces can be used to form external contact pads thereon. However, since the BAW resonators are sensitive to mechanical stress, flip-chip mounting of the filter device is preferred so that the BAW resonators face the circuit board on which the filter devices are mounted. The LC elements reacted as POG elements are not so sensitive to mechanical stress acting on them and therefore the upper surface of the POG can be used as an upper surface of the filter device.

According to another embodiment, all the passive LC elements are formed adjacent to the BAW resonators on the same surface of the substrate.

In those embodiments which require a stacked array of passive LC elements and BAW resonators, the interface between the two types of elements is sensitive to the quality and other characteristics of the device. It is preferred that a planar dielectric layer be disposed at the interface between the multilayer metallization and the BAW resonator layer. A preferred dielectric is a silicon oxide layer that can be easily polished to make the surface as flat and uniform as possible.

In one embodiment, all external contact pads of the filter circuit are on the upper surface of the BAW resonators, i. H. the surface of the layer containing the BAW resonators arranged.

According to one embodiment, the LC elements comprise a metal-insulator-metal capacitor as a series impedance element and a coil as a parallel impedance element of the branch-type or cross-type. These elements are preferably formed in different metallization levels of the POG arrangement and arranged. The coil may be implemented as a planar coil or a three-dimensional coil. While the planar coil may be formed within a metallization level, the three-dimensional coil includes half-windings in two different metallization levels interconnected by vias through the separating dielectric. In these cases, parts of the three-dimensional coil may be formed in the same metallization plane as the capacitor.

While the LC elements are embedded in a dielectric, the BAW resonators require some sort of protection from mechanical impact and mass loading. A preferred method is a thin film package that provides an air gap above the BAW resonator by depositing and patterning a sacrificial layer, covering the sacrificial layer with a rigid cap layer, and removing the sacrificial layer through an opening of the cap layer to leave a void having an air gap between the cap layer and the upper surface of the BAW resonators, i. H. the upper electrode of the resonators.

• 1 zeigt ein Blockdiagramm einer Abzweigtypanordnung aus Resonatoren;
• 2 zeigt ein Blockdiagramm eines Filterschaltkreises, der LC-Elemente und akustische Resonatoren umfasst;
• 3 zeigt ein Blockdiagramm eines Filterschaltkreises aus z. B. akustischen Resonatoren in einer Kreuzgliedstruktur;
• 4 zeigt einen schematischen Querschnitt durch ein Hybridfilter gemäß einer ersten Ausführungsform der Erfindung;
• 5 zeigt einen schematischen Querschnitt einer zweiten Ausführungsform;
• 6 zeigt einen schematischen Querschnitt durch ein Hybridfilter gemäß einer dritten Ausführungsform;
• 7 zeigt einen ausführlicheren Querschnitt durch ein Hybridfilter gemäß einer ersten Ausführungsform; und
• 8 zeigt einen schematischen Querschnitt durch eine vierte Ausführungsform, bei der der Resonatorabschnitt und die LC-Elemente auf entgegengesetzten Oberflächen des Substrats angeordnet sind.

In the following, the invention will be explained in more detail with reference to specific embodiments and the accompanying figures. The figures are drawn only schematically and not to scale. Therefore, some details may be exaggerated for better understanding.

• 1 shows a block diagram of a ladder type arrangement of resonators;
• 2 shows a block diagram of a filter circuit comprising LC elements and acoustic resonators;
• 3 shows a block diagram of a filter circuit of z. B. acoustic resonators in a cross-member structure;
• 4 shows a schematic cross section through a hybrid filter according to a first embodiment of the invention;
• 5 shows a schematic cross section of a second embodiment;
• 6 shows a schematic cross section through a hybrid filter according to a third embodiment;
• 7 shows a more detailed cross section through a hybrid filter according to a first embodiment; and
• 8th shows a schematic cross section through a fourth embodiment, wherein the resonator portion and the LC elements are arranged on opposite surfaces of the substrate.

1 shows a schematic block diagram of a ladder-type array of resonators. The resonators may be implemented in any technique, for example as acoustic resonators, such as BAW or SAW resonators. The ladder type structure may also include LC resonators, ie, a series connection of a capacitor and a coil.

The branch type structure comprises a number of base sections BS _LT and each base section comprises at least one series resonator BRs and a parallel resonator BR _P , Such basic sections BS _LT may be connected in series in a number necessary to achieve a desired filtering function and selectivity. It is possible to realize a number of different filter functions in such a ladder type arrangement of resonators. Examples are band-pass, high-pass and low-pass and combined filters such as an extractor, duplexer or multiplexer. Series resonators BRs belonging to adjacent base sections may become a common series resonator BR _S combined and parallel resonators BR _P can also be combined if they are directly adjacent to different base sections BS _LT belong.

2 Figure 12 is a schematic block diagram of a filter comprising LC elements and acoustic resonators. Only a fragmentary portion of a possible filter structure is shown including a series impedance element IEs and a parallel impedance element IEp formed as a series capacitor and a parallel coil. These LC elements or impedance elements IE are connected in series with one or more base sections of an acoustic branch type structure. The figures show a series BAW resonator BR _S and a parallel BAW resonator BR _P , A real filter circuit may comprise a large number of impedance elements IE or resonators which may be connected together in an arbitrary combination. Such a filter circuit need not be separated into a section comprising only LC elements and a section comprising only acoustic resonators, but may also comprise alternating sections of LC elements and acoustic resonators.

3 shows a schematic block diagram of a cross-member structure, the acoustic resonators BR can be implemented. A cross-member arrangement comprises two row signal lines with arranged in it series resonators BR _S , Parallel branches connect both signal lines in a crossover arrangement with each other. A parallel resonator BR _P is arranged in each parallel branch. Therefore, a base section includes BS _LC the cross-member arrangement two series resonators BR _S and two parallel branches, each having a parallel resonator BR _P include. The cross member structure may include a number of such base sections BS _LC to achieve a desired filtering function.

BAW resonators used in a filter circuit as in any of 1 to 3 can be cascaded to improve the power resistance thereof. Cascading means a series connection of two or more resonators that behave as a single resonator. This requires adjusting the resonator area to keep the static capacitance Cs constant when a BAW resonator is cascaded. A dual-cascaded resonator includes two series-connected resonators, each having a double area of a normal non-cascaded resonator.

4 FIG. 12 shows a more detailed but still schematic representation of a first embodiment thereof, as a combined filter of the block diagram of FIG 2 can be realized in a common arrangement. The resulting hybrid filter is shown with only a few elements to show a principal structure of the stacked array comprising different types of elements.

On a substrate SU which is preferably a flat glass substrate, first LC elements are formed and embedded in a first dielectric DE1. In the figure, the LC element is a metal-insulator-metal capacitor MIM implemented, consisting of a first metal structure passing through a dielectric layer DL is covered, and another metal structure as a second capacitor electrode is made.

Above the first dielectric DE1, a second metallization level ML2 is formed, structured and embedded in a second dielectric DE2. An element structured in the second metallization plane may be the upper one. Electrode of the capacitor MIM his. Further, a coil IND structured in the second metallization ML2. To form a planar coil IND For example, a single mask step is used to pattern the second metallization level accordingly.

The metal structures may be made of Al or AlCu alloy. The dielectric layer DL may be an oxide such as silica.

Above the first dielectric DE1, a second metallization level ML2 is formed, structured and embedded in a second dielectric DE2. An element structured in the second metallization plane may be the upper electrode of the capacitor MIM his. Further, a coil IND structured in the second metallization ML2. To form a planar coil IND For example, a single mask step is used to pattern the second metallization level ML2 accordingly.

Structuring a metallization level ML can be done by first forming and patterning a photoresist mask and then depositing a metal in areas exposed by the photoresist mask. Deposition of metal may be accomplished by plating a metal onto a seed layer that extends over the entire surface of the substrate SU for the first metallization level or on the first dielectric DE1 or a higher stack level of a dielectric is applied. After the plating step, the photoresist mask is removed, exposing any remaining seed layer areas, which are then removed as well.

A three-dimensional coil (not shown in the figure) IND must be formed within two adjacent metallization levels. One of them may be the first metallization level ML1.

To connect the two metallization levels ML1, ML2 to each other, a respective metallization in the lower metallization level ML1 is exposed by forming an opening in the upper surface of the first dielectric DE1. Structures of the second metallization level ML2 deposited thereon can now contact respective structures in the first metallization level ML1. All structures which do not have to have an electrical inter-level connection are isolated from each other by the first dielectric DE1.

Above the array of LC elements with at least two levels is an acoustic section AS with BAW resonators BR arranged in a branch type or cross member structure to form a hybrid filter. The BAW resonators BR can be formed in any known technique, such as SMR resonators or as FBARs on a membrane.

The figure does not show an exact filter circuit, but only gives the presence of a respective number of BAW resonators BR on. Furthermore, various filter functions can be used as described above in connection with 1 mentioned to be realized with such a hybrid filter.

In 4 No interconnection between the different stacked layers is shown. Necessary interconnections can be formed as vias. Furthermore, a dielectric layer may be provided at the interface between the acoustic section AS and the uppermost dielectric layer DE2. The hybrid filter may be more than the depicted two dielectric layers DE include.

5 shows a very different arrangement of LC elements and acoustic resonators, which are called BAW resonators BR are formed. Here, both types of elements are directly on the same surface of a common substrate SU arranged. This means, starting from an LC arrangement on a glass substrate, for example according to the known POG structures, that acoustic resonators BR on the remaining free surface of the substrate SU are arranged. Therefore, the passive impedance element section is adjacent PES directly to an acoustic resonator section BRS. Again, the passive impedance element section PES two or more metallization levels ML include.

6 shows a schematic cross section through a third embodiment. The hybrid filter starts with a substrate SU on which an arrangement of BAW resonators BR is formed. The resonators may be encapsulated in a thin film package or provided with an upper acoustic mirror deposited on top of the BAW resonators. Both alternatives are for protecting the acoustic resonator section from a mass load induced by further layers deposited on the acoustic resonator section. In both cases, a planarization step is preferred prior to depositing any other element on the resonator section. Planarization can be accomplished by depositing a dielectric of sufficient thickness that can be planarized by, for example, chemical mechanical polishing. Then, LC dielectric elements implemented, for example, as POG elements according to the method disclosed in the above-mentioned US patent application can be formed.

7 shows a schematic cross section through a hybrid filter, wherein the passive elements and the acoustic resonators are shown in more detail. The structure corresponds to the first in 4 shown embodiment and begins with a known POG arrangement on a glass substrate SU , The multilayer metallization MLM may include more than the two illustrated levels ML1 and ML2. The POG elements form part of a filter circuit having the hybrid characteristics. The first metallization level ML1 is embedded in a first dielectric DE1. The first metallization level ML1 comprises at least one lower electrode of a MIM Capacitor through a dielectric layer DL is covered, which is usually different from the embedding dielectric DE1. Furthermore, the first metallization level ML1 parts of a coil IND and connecting in-plane tracks to complete the interconnection of the LC elements.

On the first dielectric DE1, a second metallization level ML2 is provided around at least the upper electrode of the capacitor MIM and at least parts of a coil IND to include. A second dielectric DE2 covers all structures of the second metallization level ML2, thereby embedding it. Interconnections between the two metallization levels ML1 and ML2 can be provided by exposing metal structures of the first metallization level ML1 by removing respective portions of the first dielectric DE1 in an area where the interconnection is desired. In the figure, the first two structures from the left of the second metallization level ML2 are shown as being in contact with a respective structure of the first metallization level ML1.

On the upper surface of the second dielectric DE1 or on the upper surface of the uppermost dielectric (there are only two layers of the dielectric DE a planarized dielectric layer is arranged. This layer may be formed as a silicon dioxide layer.

The acoustic section AS includes an acoustic Bragg mirror AT THE and BAW resonators BR of the SMR type. Such a Bragg mirror consists of layers which alternately have a high or low acoustic impedance. The high impedance layers of the acoustic mirror AT THE may include metals and therefore must be patterned to avoid coupling adjacent capacitors. The low impedance mirror layers are usually formed of a dielectric, such as an oxide.

On the acoustic mirror AT THE is a lower electrode BE the BAW resonator formed and structured. On the lower electrode BE is a piezoelectric layer PL applied, which remains throughout and does not need to be structured. On the piezoelectric layer PL is the upper electrode TE formed and structured. The structuring of the lower electrode BE and the upper electrode TE leads to an integrated interconnection or interconnection of the BAW resonators BR , which are indicated in the figure by the corresponding equivalent symbols.

On the right side of the figure is an electrical interconnect ICN schematically represented by metal structures, at least through the acoustic mirror AT THE and through at least one layer of a dielectric DE go through it. On the interconnect ICN is a contact pad CP formed for external electrical connection of the hybrid filter. The hybrid filter can by means of a bump BU For example, be connected to an external circuitry, such as a printed circuit board.

8th shows a schematic cross section through a fourth embodiment of the invention. The fabrication of the arrangement of this embodiment may begin with a BAW wafer comprising a filter circuit of acoustic resonators BR includes. The resonators BR can on an insulating substrate SU , preferably a glass substrate, are formed. On the opposite surfaces of the substrate may be formed a multilayer metallization including interconnected LC elements implemented as POG elements according to the previous examples and embodiments.

External contact of the hybrid filter may be through contact pads on the POG section or, alternatively, on the acoustic resonator section containing the BAW resonators BR includes, be formed. The LC elements and the BAW resonators BR are interconnected by vias (not shown in the figure) through the substrate.

The invention has been explained by a limited number of examples only and accordingly is not limited to these examples. The invention is defined by the scope of the claims and may differ from the embodiments provided.

Such further embodiments may include further details not shown in the presented embodiments. Further, the hybrid filter may include any circuit of LC elements and BAW resonators that provide a desired filtering function.

LIST OF REFERENCE NUMBERS

AM

(unterer) akustischer Spiegel

ARS

Akustikresonatorabschnitt

AS

Akustikabschnitt

BE

untere Elektrode

BR

BAW-Resonator

BS_LC

Basisabschnitt einer Kreuzgliedanordnung

BS_LT

Basisabschnitt einer Abzweigtypanordnung

BU

Kontakthügel

CP

externes Kontaktpad

DE

Dielektrikum

DL

dielektrische Schicht, z. B. eine Siliciumoxidschicht

ICN

Zwischenverbindung / Via

IE_P

Parallelimpedanzelement

IE_S

Reihenimpedanzelement

IE_S, IE_P

LC-Element

IND

Spule

MIM

MIM-Kondensator

ML

Metallisierungsebene

MLM

Mehrlagenmetallisierung

PES

Passivimpedanzelementabschnitt

PL

piezoelektrische Schicht

SU

Substrat

TE

obere Elektrode

Filter circuit
conductor path
Upper acoustic mirror

AT THE

(lower) acoustic mirror

ARS

Akustikresonatorabschnitt

AS

acoustics section

BE

lower electrode

BR

BAW resonator

BS _LC

Base portion of a cross member assembly

BS _LT

Base portion of a branch type arrangement

BU

bump

CP

external contact pad

DE

dielectric

DL

dielectric layer, e.g. B. a silicon oxide layer

ICN

Interconnection / Via

IE _P

Parallel impedance element

IE _S

Series impedance element

IE _S , IE _P

LC element

IND

Kitchen sink

MIM

MIM capacitor

ML

metallization

MLM

multilayer metallization

PES

Passive impedance element portion

PL

piezoelectric layer

SU

substratum

TE

upper electrode

Claims (10)

Hybrid filter, comprising: a substrate (SU) a filter circuit having a ladder-type or cross-link array of series and parallel impedance elements selected from BAW resonators (BR) and LC elements and disposed on the substrate, wherein the LC elements comprise a metal-insulator-metal capacitor (MIM) and a coil (IND), the LC elements are formed from a multilayer metallization (MLM), each metallization level (ML) of the LC elements is embedded in a dielectric (DE), LC elements, which are formed in the same metallization plane, are electrically connected by conductor tracks, - LC elements, which are formed in different Metallisierungsebenen, by Vias (ICN) are interconnected. Hybrid filter according to the preceding claim, wherein the substrate (SU) is a glass substrate. Hybrid filter according to the preceding claim, wherein the LC elements (MIM, IND) are implemented as passive elements on glass. Hybrid filter according to the preceding claim, - wherein the LC elements are formed on the glass substrate (SU), wherein a planar dielectric layer is arranged above the LC elements, - wherein the BAW resonators (BR) are formed directly on the planar dielectric layer. Hybrid filter after one of Claims 1 to 3 wherein the BAW resonators (BR) are formed on the surface of the substrate (SU), each BAW resonator comprising at least one upper acoustic mirror, the LC elements being formed as passive elements on the upper acoustic mirror. Hybrid filter after one of Claims 1 to 3 wherein the LC elements and the BAW resonators (BR) are formed on opposite surfaces of the substrate (SU). Hybrid filter after one of Claims 1 to 3 wherein the LC elements and the BAW resonators (BR) are formed adjacent to each other on the same surface of the substrate (SU). Hybrid filter after Claim 4 wherein the planar dielectric layer comprises a polished silicon oxide layer. Hybrid filter according to one of the preceding claims, wherein all external contact pads (CP) of the filter circuit are arranged on the upper surface of the BAW resonators (BR). Hybrid filter according to one of the preceding claims, wherein the filter circuit comprises a metal-insulator-metal capacitor (MIM) as a series impedance element (IE _S ) and a coil (IND) as a parallel reactance element (IE _P ) of the branch-type or cross-link type arrangement, wherein the BAW resonators (BR) and the LC elements (MIM, IND) are interconnected by vias.

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